1. The Field of the Invention
The present invention is related to a case and closure design for a gun-launched rocket More particularly, the present invention relates to an insensitive munitions system employing one or more passages and an internal compression chamber system which collectively accommodates gas pressures inherent in gun-launched rocket systems in a manner allowing for a thinner rocket motor case cylinder and increased propellant volume beyond what prior art permits. These systems may be used in combination or separately.
2. Technical Background
Many launchable projectiles or rockets are comprised of a forward end, including guidance and munitions, and an aft end rocket motor. These two elements can be formed together, with a common outer casing, or they can be separately formed and subsequently joined together. This joining can occur immediately prior to use, in which case the two elements must be separately stored or they can be joined together for storage purposes and be ready for immediate use.
During pre-launch storage when a rocket motor is ignited inadvertently by external heating, such as a spilled fuel fire, it will become propulsive before being properly aimed. When inadvertent ignition is caused by fragment impact that produces unplanned nozzle outlets, the motor may become wildly propulsive in undesired directions. And when such events produce unplanned increases of propellant burning surface area, excessive pressurization may increase the hazard to nearby personnel and property. In light of these dangers, many of today's weapon systems must satisfy certain insensitive munitions (IM) requirements focused on safe storage capabilities.
Where rocket motors are stored separately, one way that rocket motors meet IM requirements is by venting the internal pressure caused through inadvertent ignition of the propellant by discharging either the forward or aft closure of the case cylinder. This allows the propellant to burn through a now open end without generating substantial thrust in any direction and without the threat of the rocket motor exploding and spraying burning propellant and metal case cylinder fragments in numerous directions.
The prior art teaches the use of dual paths for load transfer between features of either closure or between the closure and the motor case cylinder. One such load path may be sized to accommodate relatively small loads expected during transportation and handling prior to gun launch, and the other to accommodate much larger loads encountered during launch or during rocket motor operation. Focusing on shells which may or may not include rocket motors, Hickey teaches in U.S. Pat. No. 4,557,198 (1985) use of shear pins or locking rings arranged so that the high load capability load path is armed by axial acceleration during normal launch (with or without spin-up of the round) which also disarms the low capability load path. Boissiere, in U.S. Pat. No. 5,337,672 (1994), teaches arming of the high capability load path and disarming the low capability load path by gas pressures produced by the round itself. Dolan, in U.S. Pat. No. 4,597,261 (1986), Panella in U.S. Pat. No. 3,887,991 (1975), Tate in U.S. Pat. No. 5,036,658 (1991), Koontz in U.S. Pat. No. 5,155,298 (1992), Ellingsen in U.S. Pat. No. 5,311,820 (1994), and Cherry, in Statutory Invention Registration H1144 (1993), teach use of thermally activated devices of similar intent. Further, Malamas in U.S. Pat. No. 4,991,513 teaches use of a vent system that is closed by spin-up at launch.
As will be shown below, the safe expulsion of either closure can also be accomplished through the use of a low shear retaining means positioned between components of the closure or between the closure and the rocket motor case cylinder and a high capability load path that is disarmed until subjected to gun pressure. Should the propellant be inadvertently ignited, the low shear retention means will shear under relatively low internal pressure and allow the entire closure, or a portion thereof, to disengage from the case cylinder. Thus, the internal pressure induced by inadvertent ignition will vent without the dangers associated with premature propulsion or explosion.
As will further be shown below, advantageous use of gun pressure to arm a high capability load path may be combined with a further use to diminish the inert--non-propellant--weight of the gun-launched projectile.
Rocket motors are used to propel a payload. The amount of payload that can be propelled, as well as how fast and how far, are substantially affected by the weight--or mass--of the rocket motor components themselves. For the most demanding mission performance requirements, rocket motor designers focus intently on minimizing the inert weight of the motor itself.
Some rocket motors are intended for use in gun-launched projectiles, either in a "sustain" mode that diminishes losses of velocity or range due to one friction, or in a "boost" mode that increases the velocity beyond what is imparted by the gun pressure and gun tube length. In the prior art relating to such applications, the high pressures and accelerations imparted to the projectile during gun launch demand significant increases in the rocket motor inert weight--beyond what would otherwise be needed for motor operation alone.
In addition, the gun accelerations may threaten the integrity of the propellant charge itself unless great care is exercised over its configuration and means of support. Accelerations imposed within the gun tube upon gun-launched projectiles are hundreds--even thousands--of times larger than those for rocket launched projectiles.
There are two types of solid propellants for rockets. In one type, which consists of compressed powder, virtually the entire cumulative surface area of all the particles is available for combustion immediately upon ignition. In the other type, the fuel and oxidizer particles are bound together by a rubber matrix forming a composite which becomes solid after a cure protocol of thermal or other character. With the composite propellants, the burning surface area is readily controlled by adjusting the shape of the solid material and the burn rate features of the formulation. During the burn of a compressed powder propellant, vastly higher operating pressures prevail than during burn of a like quantity of composite propellants. It follows that compressed powder propellants are generally used only where the gun barrel can be used to withstand the high pressures. When the propellant is to burn after the rocket leaves the gun, it is generally a composite propellant.
The thermal expansion characteristic of composite solid propellants is typically an order of magnitude larger than that of the enclosing or containing structure. A 100.degree. F. operating temperature range therefore produces a volume change of about 2%. Unless the configuration and support arrangement allow deformations to occur, thermal stresses in the propellant may cause fractures, undesired increases of burning surface area, and disasters upon ignition. Common provisions for thermal expansion include a central axial perforation for propellant grains bonded on their outer circumferential surfaces to cylindrical vessels, and completely free outer surfaces for propellant grains bonded at either their forward or aft ends to vessel closure features.
The tensile and shear strengths and elastic moduli of typical propellants are minuscule in comparison with the containing structure. For this reason, departures from a hydrostatic stress state during gun launch are accompanied by large deformations. At high forward acceleration, the propellant grain tends to completely fill the available volume of the aft end of the containing vessel. For a free-standing grain supported at its aft end with an unbonded circumferential surface, a 10,000 g environment and a 17 inch axial length lead to a 10,000 psi hydrostatic compression at the support.
During gun launch, alternatives to the aft end support arrangement for the propellant grain are grave threats to its integrity. Indeed, at acceleration levels typical of gun launches, neither the bonded circumferential surface of an axially perforated propellant grain nor an unperforated grain with a bonded forward end is stiff enough to eliminate the aft end support mode unless there is a great deal of empty space within the motor.
It follows that virtually the entire force that accelerates the propellant grain during gun launch is applied by direct bearing through its aft end. It also follows that in a 17 inch long propellant grain of a typical propellant, the bearing stress will be 10,000 psi at an acceleration of 10,000 g's. It follows also that the circumferential surface of the propellant grain will expand to fill the cylinder, imposing a radial pressure varying with depth (hydrostatically) from 10,000 psi at the aft end to zero at the forward end.
Therefore, during gun launch, the case cylinder experiences tension in the hoop direction due to internal pressure that may well be several times larger than the operating pressure later in flight, when the propellant burns. Moreover, during gun launch, the axial force needed to accelerate the payload, forward of the rocket motor, must be carried around the propellant grain, by axial compression in the rocket motor case, which must be proportioned so that buckling does not occur.
The buckling load for an axially compressed thin cylinder depends on its radius, thickness and length, and upon a single material property, the modulus of elasticity, at the actual imposed effective stress level. When the material "yields", the modulus decreases from the initial value, Young's modulus, to zero eventually (for ductile metals). The effective stress (von Mises yield criterion) under the combined hoop tension and axial compression stress state imposed during gun launch can be as much as 73% above the hoop or axial stress, whichever is larger. Effectively, the material yields under the mixed tension and compression condition at a far lower stress level than if either stress were acting alone, and the modulus of elasticity--and the buckling load--are thereafter much reduced. Thus, the thickness needed to assure a suitable safety factory is much higher than would be deduced for either the internal pressure or axial force alone.
In recent years, efforts to overcome the above-described behaviors of both the propellant and the case cylinder have turned to admitting the gun pressure to the interior of the rocket motor case. Paget, in U.S. Pat. No. 3,349,708, discloses several embodiments of a firearm launched rocket projectile that contains a solid fuel and permits gun bore gases to enter within the aft end. Paget is concerned with keeping the casing thin and suggests this can be achieved using passageways that distribute equal gun bore pressures on both the inside and outside of the projectile. The gun propulsion gases must also come into contact with the solid fuel used by Paget as that is the source of fuel ignition. Pins or springs hold passageways open to initially receive gun propulsion gases with such passageways being closed once the projectile exits the bore of the arm.
Another approach has included filling the internal free volume with a soft-extrudable fluid, rubber or otherwise. Thus, the fluid moves in and out of the rocket motor, through the nozzle throat orifice, as the storage temperature changes, and the nozzle exit cone becomes a reservoir for excess fluid. The fluid is ejected when the propellant is ignited.
Admitting the gun pressure to the interior of the rocket motor, with the fluid void-filler, has both obvious and subtle implications. Among the obvious is that unless the exterior surface of the rocket motor is also exposed to gun pressure, the case cylinder will have to accommodate as much as 60,000 psi internal pressure, or more--an order of magnitude above the usual range of rocket motor operating pressures. To accomplish this, the obturator, the sliding seal between the projectile and the gun tube that prevents the gun pressure from escaping around the projectile, is moved from the aft to the forward end of the rocket motor. It follows that, for the quasi-static situation at maximum acceleration, the differential pressure across the case cylinder wall is external pressure of varying magnitude, reflecting the hydrostatic gradient in the propellant grain. Further, the axial compression in the case cylinder disappears because the accelerating force for the payload is applied directly to the forward closure.
The subtle implications reflect the dynamic situations as the gun pressure rises rapidly upon ignition, and as the gun pressure disappears when the obturator passes out of the gun bore. At the outside, because the orifice into the rocket motor is quite small, the intensity of the gun pressure applied to its interior lags the pressure intensity applied to the exterior. This threatens to buckle the case if the duration of the lag is large enough. Also when the obturator clears the gun bore, the small nozzle orifice prevents an instantaneous drop of internal pressure after the external pressure disappears. This threatens to burst the case unless it has been made thick enough to withstand the gun pressure level--acting alone--that prevails immediately before the obturator clears the gun bore.
Gun-launched rocket motors are obviously limited in their outside diameter by the size of the gun bore from which they are fired. Thus, given usual propellants and rocket motor nozzles, greater range or velocity is achieved for the projectile by configuring the rocket motor such that it can hold a maximum amount of propellant.
The volume of propellant in gun launched rocket motors is maximized when the interior diameter of the rocket motor case cylinder is maximized by making the case cylinder as thin as possible. However, the case cylinder must be thick enough to accommodate gun launch loads, and, when gun pressure is allowed within the case cylinder, the pressure differentials between the inside and outside of the case cylinder, not only at maximum levels, but as the gun pressure rises early during launch and falls as the rocket motor exits the gun bore. Rocket motors designed according to the prior art must therefore survive gun launch loadings that are frequently far more severe than the later loadings during rocket motor burn. This requires thicker structures which diminish the volume available for propellant, and which increase the inert weight of the motor, thereby diminishing the attainable range or velocity of the projectile.
Thus, an advancement over the prior art is achieved by introducing rocket motor configuration features that diminish the net loads that the rocket motor case cylinder must be designed to withstand during gun launch, thereby diminishing the inert weight and increasing the available propellant volume while also providing an insensitive munitions case and closure design for a gun-launched rocket motor.
Such gun-launched rocket motor configuration features are disclosed and claimed herein.